Few things are as embedded in Maine's culture — or its mud — as clams, and an environmental group thinks the key to saving the shellfish might be growing a different kind of bivalve along the state's coast. Manomet, based in Plymouth, Massachusetts, is proposing the shellfish shift as a way to beat predators that plague Maine's clam diggers. Seafood lovers have sought Maine's softshell clams in chowders and clam rolls for decades, but wild harvesters are collecting fewer of those clams, in part because of the spread of crabs and worms that prey on them.